Ecology

Algal Bloom

A rapid increase in algae population in a lake, often visible as green discoloration or surface scums, triggered by warm temperatures and excess nutrients.

What It Means for Your Lake

An algal bloom is a rapid proliferation of phytoplankton that causes visible changes in lake water color, clarity, or surface conditions. Blooms occur when the combination of warm water temperatures (typically above 68 degrees Fahrenheit), abundant nutrients (especially phosphorus), calm wind conditions, and long daylight hours create ideal growth conditions. In Minnesota and Wisconsin, algal blooms are most common from July through September. Not all algal blooms are harmful, blooms of green algae (Chlorophyta) and diatoms are generally nuisance issues that reduce water clarity and aesthetics but do not produce toxins. However, blooms dominated by cyanobacteria (blue-green algae) can produce potent hepatotoxins (microcystins) and neurotoxins (anatoxins) that are dangerous to humans, dogs, livestock, and wildlife. Harmful algal blooms (HABs) are identified by their appearance: they look like green paint, pea soup, or floating mats of blue-green material on the water surface. During a HAB event, the Minnesota Pollution Control Agency and Wisconsin DNR recommend avoiding all body contact with the water, keeping pets and children away from shorelines, and not consuming fish from the affected lake. Algal blooms have increased in frequency and intensity across the Upper Midwest over the past three decades due to a combination of rising temperatures, increased extreme rainfall events that wash more nutrients into lakes, and continued high phosphorus inputs from agricultural and urban sources.

Frequently Asked Questions